1. Field of the Invention
The present invention relates to a measuring device in a scanning probe microscope.
2. Description of the Related Art
Typical scanning probe microscopes include a scanning tunneling microscope (STM) and an atomic force microscope (AFM).
The STM is a surface configuration measuring device wherein an electrically conductive pointed probe is approached to an electrically conductive sample at a distance of several .ANG. and a tunnel current occurring when a bias voltage is applied across the probe and the sample is utilized. In general, in the STM, the probe/sample distance is servo-controlled so as to keep it constant when the probe scans the surface of the sample. A high-voltage driven piezoelectric element is generally employed to effect scanning and servo control of the probe.
The tunnel current is detected through a probe electrode which is attached to the piezoelectric element with an insulator member interposed. A servo voltage corresponding to the configuration of the sample surface is supplied to the piezoelectric element, so that the probe may scan the sample surface with a constant distance. An insulator member is interposed between a probe electrode and a drive electrode of the piezoelectric element to which the servo voltage is applied, and a capacity is constituted between the drive electrode and the probe electrode. Accordingly, if a servo voltage varying in the course of time is applied to the drive electrode of the piezoelectric element, electric current flows to the probe electrode, as if an AC voltage were applied to an electrode of the capacity. This prevents precise measurement.
In order to solve the above problem, Japanese Patent Application No. 2-134837 (which is based on U.S. patent application Ser. No. 07/589,491, now U.S. Pat. No. 5,083,022) proposes a method of providing a grounded girdling electrode between a probe electrode and a piezoelectric element with an insulating portion interposed, and a method of providing a girdling electrode on a piezoelectric element itself thereby grounding the piezoelectric element.
These methods are advantageous in the case of applying a constant bias voltage to a probe, or applying a modulated bias voltage to a sample, with a probe virtually grounded, thus measuring a local electrical characteristic of the sample on the basis of a tunnel current (the latter is called "Scanning Tunneling Spectroscopy (STS)"). However, if it is necessary to keep the sample at a ground potential or a fixed potential, as in the case of an in-liquid STM, a bias voltage is applied to the probe. In this case, if STS measurement wherein a bias voltage to the probe is modulated is carried out, electric current flows to the probe electrode via an insulator member interposed between the probe electrode and the girdling electrode, since the girdling electrode is grounded. Thus, the electric current is erroneously detected as a tunnel current, and exact STM measurement cannot be carried out.
On the other hand, in the AFM, a cantilever 100 .mu.m to 2000 .mu.m long, which is formed of an elastic material, has a free end portion provided with a pointed probe. The probe is approached to a sample, and an attractive force such as adsorption force or van der Waals force occurs between the atom on the probe tip and the sample surface. On the basis of the displacement of the cantilever resulting from the attractive force, a local configuration and/or characteristic of the sample is measured.
In addition to the above-described attractive force mode, the AFM has a repulsive force mode based on an inter-atomic repulsive force, based on the Pauli principle, acting between one atom of the probe tip and one atom of the sample. In the former mode, the force of 10.sup.-7 to 10.sup.-12 N is detected on the basis of the displacement of the cantilever; in the latter mode, the force of 10.sup.-7 or more is detected similarly.
In the repulsive force mode, what is to be measured by the AFM is mainly an inter-atomic force acting between only the closest atoms of the probe tip and the sample. In other words, although an attractive force acts between the atoms other than the closest atoms, the attractive force is much weaker than the repulsive force and is negligible.
On the other hand, in the attractive force mode, there occurs a force which influences the displacement of the cantilever and is other than inter-atomic force. One example is a meniscus force occurring when water molecules gather between the probe and sample.
S. Manne and P. K. Hamsma et al of the California University, Barkley proposed, as described in "Imaging metal atoms in air and water using the atomic force microscope" Appl. Phys. Lett. 56(18), Apr. 30, 1990, an effective method of putting a probe and a cantilever in water and carrying out measurement while meniscus force is prevented from affecting the probe and cantilever, generally, when a non-polarized liquid such as water is employed.
However, in an environment in which the probe and cantilever are surrounded by a polarizable insulating material, the inside and outside of the AFM apparatus are influenced by electrostatic force of a charge which occurs on the surface of the probe and cantilever since the insulating material is polarized by the presence of objects having abnormal potential energy. This prevents exact measurement.